Hard silicon-based ceramic materials are known and are used extensively in such applications as metal cutting and boring tools, metal drawing dies, wear-resistant machine parts and the like. Such silicon-based materials include monolithic and composite silicon nitride and silicon carbide materials. The service properties such as wear, high temperature and chemical resistance of such materials may be enhanced by the application of one or more thin coatings of, for example, metal carbides, metal nitrides, or ceramics Cutting tools of coated silicon nitride-based materials are disclosed in U.S. Pat. Nos. 4,416,670 and 4,431,431 (metal carbide coatings), 4,406,667 and 4,406,668 (metal nitride coatings), 4,409,003 and 4,409,004 (metal carbonitride coatings), 4,421,525 and 4,440,547 (alumina coatings), and 4,441,894 and 4,449,989 (metal carbide, nitride or carbonitride undercoatings with alumina outer coatings). Great strides have been made in improved performance of these coated substrates, for example in machining applications, by refinement of the substrate compositions and by applying various combinations of superimposed layers of coating materials.
Such high technology ceramic materials offer the potential for good performance characteristics for high temperature applications due to their mechanical strength and fracture toughness. However as components for advanced gas turbine engines, they have not demonstrated satisfactory resistance to contact stress, including sliding contact stress, and to high temperature oxidation under contact stress conditions. Oxide coatings are known to provide oxidation resistance to various materials at high temperatures. However contact stress damage at ceramic-ceramic interfaces is still a major failure mode for ceramic heat engine components. Researchers have found that oxide coatings about 50-100 microns thick of the alumina or zirconia type provide improved protection against such surface damage and loss of strength under sliding contact. Such coatings however have not shown adequate adherence under long term use in heat engine applications. Thermal barrier coatings applied by physical vapor deposition methods have been developed for diesel engine components. The coating has a bond coat about 100-200 microns thick of a metal-chromium-aluminum-yttrium composition, the metal being Fe, Co, or Ni. A protective yttria stabilized zirconia coating about 800 microns thick is applied over the bond coat. These coatings, however, for the most part are inherently porous unless applied by Electron Beam Evaporation methods.
Severe thermal cycling conditions are encountered by the components used in heat engines, where cycling between ambient temperatures and temperatures that can reach as high as 1400.degree. C. is repeatedly encountered over long periods of use. Differences in the thermal expansion coefficients of the materials of the substrate and the coating and, in multilayer coatings, of the materials of the various layers of the coating can lead to high tensile and compressive stresses in the coating. These stresses are also dependent on the thickness of the coating. Severe stresses can lead to thermal shock and to cracking and debonding of the coating under such severe conditions of use.
Further, severe localized surface stress can occur in such components due to the brittleness and hardness of the silicon based materials and the oxide coatings. The severity of the stress is largely due to the fact that the hardness of the materials prevents redistribution of the localized stress. If the localized stress exceeds the baseline strength of the material, the surface of the component will be damaged, and the strength and oxidation resistance of the component will be reduced.
The invention described herein and recited in the appended claims provides an article in which a ceramic coating of controlled graded composition and distribution is deposited on a hard silicon-based ceramic substrate, the article showing long term adherence and contact stress resistance in oxidizing environments and/or high temperature thermal cycling conditions, such as those described above.